As well known, the electric power steering apparatus are steering assisting apparatus which are constructed to activate an electric motor (steering assisting motor) as a human driver manually operates a steering wheel of a motor vehicle, to thereby assist the driver's manual steering effort. In such electric power steering apparatus, the steering assisting motor, which provides steering assist force or torque, is controlled on the basis of a steering torque signal generated by a steering torque detection section detecting steering torque that is produced on the steering shaft by driver's operation of the steering wheel and a vehicle velocity signal generated by a vehicle velocity detection section detecting a traveling velocity of the vehicle, so as to reduce manual steering force to be applied by the human driver.
For example, Japanese Patent Application Laid-Open Publication No. 2001-275325 discloses an electric power steering apparatus, where the steering torque applied to the steering wheel is delivered to an output shaft of a rack and pinion mechanism while the steering assist torque produced by the electric motor in accordance with the steering torque is delivered to a pinion shaft via a frictional transmission mechanism and worm gear mechanism. Thus, road wheels of the vehicle are steered via the rack and pinion mechanism. The disclosed electric power steering apparatus is designed to: impart a good steering feel by minimizing adverse effects of steering torque variation by the motor when the vehicle should travel straight with the motor kept deactivated; and enhance the controllability of the vehicle by efficiently enhancing the output performance of the motor. For these purposes, the electric motor comprises an annular outer stator including armature windings wound on nine or N (N is an arbitrary number equal to a multiple of nine) circumferentially-arranged poles, and an inner rotor located inwardly of the outer stator and including circumferentially-arranged permanent magnets of eight poles.
In the electric motor of the disclosed electric power steering apparatus, the armature-winding connecting line, connecting in series the armature windings, comes out of one of the armature windings, then extends to the next armature winding, adjoining the one armature winding, where it arcuately extends around (i.e., substantially straddles) a considerable or relatively great part of the outer periphery of the armature winding to reach a point of the next armature winding remote from the one armature winding (rather than a point of the next armature winding close to the one armature winding), and then connects to the further next armature winding that does not adjoin the one armature winding. The extra length substantially straddling the considerable part of the outer periphery of the next armature winding as noted above would considerably increase the total length of the armature-winding connecting line. Further, in a case where the armature-winding connecting line should connect from one armature winding of a given phase to another armature winding of the same phase that is spaced from the one armature winding with other armature windings of other phases interposed therebetween, the total length, per phase, of the armature-winding connecting line would inevitably have to be increased further due to an additional length straddling parts of the armature windings of the other phases.
FIG. 10 shows an armature-winding connecting line and a neutral-point connecting line in the electric motor of the disclosed electric power steering apparatus, and FIG. 11 is a conceptual view representatively showing salient poles of one of three phases (U phase in the illustrated example) and respective numbers of turns of the armature windings 100a-100c. The armature windings 100a, 100b and 100c of adjoining three poles are connected in series to provide a U-phase winding unit, armature windings 100d, 100e and 100f of other adjoining three poles are connected in series to provide a V-phase winding unit, and the armature windings 100g, 100h and 100i of the other adjoining three poles are connected in series to provide a W-phase winding unit.
In the conventional electric motor, as shown in a plan view of FIG. 10, the armature-winding connecting line 110, connecting in series the adjoining armature windings 100a-100c, 100d-100f, 100g-100i, comes out of one of the armature windings, then arcuately extends around (i.e., substantially straddles) a considerable part of the outer periphery of the next armature winding adjoining the one armature windings, then intersects with another armature-winding connecting line 110 to connect to the further next armature winding, as depicted within an oval in the figure. Further, the neutral-point connecting line 120 is led or positioned on the same side as the armature-winding connecting lines 110, so that layout or placement of the electric motor tends to be difficult and human operators tend to confuse the armature-winding connecting lines 110 and neutral-point connecting line 120 during assembly operations.
As further illustrated in FIG. 11, the armature-winding connecting lines 110 would inevitably intersect around the middle salient pole 101b if the armature windings are to be wound to the same number of turns on the adjoining salient poles 101a-101c. In FIG. 11, the armature windings are each shown as wound to five turns on the corresponding pole, as depicted by “(5)”. For the reasons stated above, there has been a demand for an improved electric motor where the armature-winding connecting lines 110 do not intersect and which has a winding structure that can be laid with increased ease and that can be reduced in size.